The major goals of this project are to test the hypotheses: 1) that embryonic dopaminergic tissue implanted into the putamen bilaterally in patients with advanced Parkinson's disease will provide statistically significant and clinically meaningful improvement; 2)that this improvement is not due to the psychological effect of having the surgical procedures (by having a double-blinded sham-operated group as a comparison); 3) that age is an important variable for benefit; 4) that the transplantation procedure is safe and well tolerated; and 5) that increased fluorodopa uptake in the putamen as observed by PET scan correlates with the clinical results. Forty subjects with advanced Parkinson's disease will be enrolled, 20 younger than 60 years of age, and 20 older. After baseline measurements, subjects will be randomized to transplant surgery or sham surgery, and followed every four months for one year, at which time the blind will be broken. Subjects receiving sham surgery will have the option of undergoing transplant surgery. Subsequently, all subjects will be evaluated every six months for the duration of the study.